Understanding fracture behavior of epoxy-based polymer using molecular dynamics simulation.

With a network structure formed by cross-linking process, epoxy resins possess excellent mechanical properties. Among them, SU-8 photoresist (bisphenol-A epoxy resin) is widely used as coatings in microelectronic applications. This paper reports molecular dynamics simulations of the cross-linking process of SU-8 photoresist with detailed scripts and illustrates fracture behaviors of cross-linked network with in situ visualizations of atomistic details. The epoxy molecular models with the cross-linking degree of 20%, 40%, 60% and 80% are constructed, which possess density, glass transition temperature, and mechanical properties in a good agreement with existing experimental studies on SU-8. For the models with cross-linking degree of 60% and 80%, uncoiling phenomena observed during large-strain tensile deformations are proved to be associated with significant drops of stress and the dissipation of local concentrated energy developed within the stretched structure, which indicates that the coiling of molecular chains is a critical structure for strength of epoxy molecules. The findings from this paper enrich our understandings of molecular details of SU-8 photoresist upon fracture and contribute as a useful reference to atomistic modeling database of cross-linked polymers.